Confused about redirects

I understand the best practice is typically to redirect 404s to the most relevant page. What I don’t get is when the 404s are super old (like 2-5 years), does it still matter? Basically I have a plug-in that is telling me I should redirect about 5,000 404s. I think it identifying all 404s that ever existed. From what I understand, there are two reasons to redirect 404s. First, that it’s for the user to stay on your site and second, so that crawlers can easily index your site. Am I understanding this right? Should I only redirect recent 404s, not super old ones?